This invention relates to a method of and means for maintaining a halocine in an open body of salt water.
A halocline is a zone in a body of salt water wherein a marked salinity gradient is present, the gradient being such that salinity increases with depth. As is well known, convection currents within a halocline are suppressed by the difference in vertical density. As a consequence, the presence of a halocline adjacent a surface of a body of water at a depth to which solar radiation penetrates (i.e., 1-3 meters) is accompanied by heating of the halocline and a layer of liquid therebeneath to temperatures significantly higher than the surface temperature of the body of water. In the absence of the halocline, heat absorbed below the surface of the body of water would be transferred by convection currents to the surface of the water where it would be dissipated by evaporation and long-wave radiation. Thus, a stable halocline in a shallow pond converts the pond into a solar collector permitting temperatures as high as 100.degree. C. to be obtained at a depth of about one meter below the surface.
The ability of shallow pond to act as a solar collector depends upon the stability of the halocline. Factors tending to destabilize a pond are molecular diffusion of salt along the concentration gradient, mixing of the upper layer of liquid due to wind action at the surface, and induced convection associated with heat extraction at the bottom of the pond.
An initial approach to maintaining stratification and stabilizing the halocline is disclosed in Israeli Patent No. 12561 of May 25, 1959. This patent discloses a system for continuously flushing the surface of a solar pond with fresh water and adding salt at the bottom. In order for this technique to be successful, a source of fresh water is required and the pond is not self-maintained in the sense that it requires intercession from outside the system to maintain the halocline.
A self-maintaining solar pond is disclosed in U.S. Pat. No. 3,372,691 of Mar. 12, 1968 wherein a downward vertical motion across the halocline is induced by flash evaporating liquid drawn from a heated layer of liquid below the halocline to obtain fresh water and a solution whose concentration exceeds that of the heated layer liquid. Some of the concentrated solution is returned to the heated layer of liquid, and make-up water to maintain the level of the pond is added at its surface. A solar collector operating on this principal is termed a descending or falling solar pond because of the downward motion across the halocline. Such motion can be adjusted, in theory, to counter the diffusion flux of salt in the pond by controlling the rate at which flash evaporation takes place.
It can be shown analytically that as the downward motion across the halocline increases, the slope of the halocline in its upper region (i.e., near the surface of the pond), becomes quite large, and in fact, the salinity profile is asympotic to the vertical near the surface. As a consequence, the salt gradient near the top of the halocline will be very small and hence highly unstable when heated by more than a few degrees. Furthermore, wind mixing near the surface will completely destroy small salt gradients.
The situation in the upper portion of the halocline can be improved if most of the fresh water produced by the flash evaporator is returned to the pond. However, in arid zones, fresh water is scarce and its use in this manner is wasteful. Moreover, brackish or sea water is often available as make-up water to compensate for evaporation, but the salt from such water will slowly accumulate and eventually destroy the stratification. In order to maintain the balance, downward movement across the halocline can be increased to not only compensate for molecular diffusion, but to bring salt to the bottom of the pond where it is available for crystalization from the flash evaporator. It can be shown that as the salinity of the make-up water increases, an even greater vertical movement is needed, and such greater vertical movement further reduces the already small gradient in the upper half of the halocline. As a consequence, the pond under these conditions will be very unstable against wind mixing and induced mixing due to circulation associated with heat extraction.
It is therefore an object of this invention to provide a new and improved method of and means for maintaining a halocline in an open body of water wherein the disadvantages of the prior art, as outlined above, are reduced or substantially overcome.